Electromagnetic Properties of Aluminum-Based Bilayers for Kinetic Inductance Detectors

• Published in: IEEE transactions on applied superconductivity Vol. 33; no. 5; pp. 1 - 6
• Main Authors: Wang, G., Barry, P. S., Cecil, T., Chang, C. L., Li, J., Lisovenko, M., Novosad, V., Pan, Z., Yefremenko, V. G., Zhang, J.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.08.2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Aluminum; Bilayers; complex conductivity; Conductivity; density of states; Detectors; Electromagnetic properties; Elementary excitations; Energy gap; Impedance; Inductance; Iridium; Mathematical analysis; Phonons; Photons; Proximity effect; Sensors; Superconducting microwave devices; Superconducting transition temperature; Superconductivity; Surface impedance; Temperature measurement; Thickness; Thin films; Usadel equations
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2023.3245059

The complex conductivity of a superconducting thin film is related to the quasiparticle density, which depends on the physical temperature and can also be modified by external pair breaking with photons and phonons. This relationship forms the underlying operating principle of Kinetic Inductance Detectors (KIDs), where the detection threshold is governed by the superconducting energy gap. We investigate the electromagnetic properties of thin-film aluminum that is proximitized with either a normal metal layer of copper or a superconducting layer with a lower <inline-formula><tex-math notation="LaTeX">T_{C}</tex-math></inline-formula>, such as iridium, in order to extend the operating range of KIDs. Using the Usadel equations along with the Nam expressions for complex conductivity, we calculate the density of states and the complex conductivity of the resulting bilayers to understand the dependence of the pair breaking threshold, surface impedance, and intrinsic quality factor of superconducting bilayers on the relative film thicknesses. The calculations and analyses provide theoretical insights in designing aluminum-based bilayer kinetic inductance detectors for detection of microwave photons and athermal phonons at the frequencies well below the pair breaking threshold of a pure aluminum film.